Adding the ability to use a layout for your html templates. (#70)

* Adding the ability to use a layout for your html templates. Fixes #69

* apparently I can't do a comment here...

spec/email_spec.cr

@@ -73,6 +73,11 @@ private class EmailWithCustomAttributes < Carbon::Email
   end
 end
 
+private class EmailWithLayout < BareMinimumEmail
+  templates html
+  layout custom_layout
+end
+
 describe Carbon::Email do
   it "can build a bare minimum email" do
     email = BareMinimumEmail.new
@@ -135,4 +140,10 @@ describe Carbon::Email do
 
   it "normalizes recipients" do
   end
+
+  it "includes a layout" do
+    email = EmailWithLayout.new
+    email.html_body.should contain "Email Layout"
+    email.html_body.should contain "Email body"
+  end
 end

spec/templates/custom_layout/layout.ecr

@@ -0,0 +1,3 @@
+<h1>Email Layout</h1>
+
+<%= content %>

spec/templates/email_with_layout/html.ecr

@@ -0,0 +1 @@
+<p>Email body</p>

src/carbon/email.cr

@@ -19,20 +19,39 @@ abstract class Carbon::Email
 
   def text_body; end
 
+  def text_layout(content_io : IO); end
+
   def html_body; end
 
+  def html_layout(content_io : IO); end
+
   getter headers
 
   macro inherited
     macro templates(*content_types)
       \{% for content_type in content_types %}
         def \{{ content_type }}_body : String
-          io = IO::Memory.new
-          ECR.embed "#{__DIR__}/templates/\{{ @type.name.underscore.gsub(/::/, "_") }}/\{{ content_type }}.ecr", io
-          io.to_s
+          content_io = IO::Memory.new
+          ECR.embed "#{__DIR__}/templates/\{{ @type.name.underscore.gsub(/::/, "_") }}/\{{ content_type }}.ecr", content_io
+          \{{ content_type }}_layout(content_io) || content_io.to_s
         end
       \{% end %}
     end
+
+    # Specify an HTML template layout
+    #
+    # ```
+    # templates html
+    # layout email_layout
+    # ```
+    macro layout(template_name)
+      def html_layout(content_io : IO)
+        content = content_io.to_s
+        layout_io = IO::Memory.new
+        ECR.embed "#{__DIR__}/templates/\{{ template_name.id }}/layout.ecr", layout_io
+        layout_io.to_s
+      end
+    end
   end
 
   @headers = {} of String => String